Quantum teleportation, which utilizes entanglement, is a critical technique for enabling quantum relays and quantum repeaters. In quantum teleportation an incoming qubit (e.g., a single photon), typically in an unknown quantum state, is combined with one half of a two-qubit (e.g., two-photon) entangled pair, and a Bell-state measurement is performed. The result of this measurement is then fed forward to apply an appropriate unitary transformation to the third qubit (that was originally part of the entangled pair). Upon application, the unitary transformation sets the third (outgoing) qubit's state to that of the unknown (incoming) qubit.
However, to maximize and extend the reach, additional teleportations must be made, each of which requires the creation of another entangled pair. In the case of an optical fiber link or optical fiber network, photons are normally used as qubits for transmission of a quantum state between distant locations. Two-photon entangled pairs at quantum relay or quantum repeater sites can be used to enable teleportation. This technique can relay the unknown state of an initial photonic qubit farther along a fiber or fiber network than would be feasible by simply transmitting the original qubit over the fiber or fiber network. In addition, this architecture may be used to swap entanglement if the first qubit is entangled with an additional qubit. To maximize the distance between relay or repeater stages, additional entangled-photon pairs must be created at widely separated locations.
To avoid degrading the quality of the transmission of information, it is desirable that the entangled photon pair generated at a given relay or repeater is indistinguishable from the entangled photon pair generated at the previous relay or repeater site. One approach to enhance the indistinguishability is to use the same laser pump to create both pairs. However, because most implementations utilize an ultrashort laser pump to create entangled photons, this approach normally requires the two pairs to be created in close spatial proximity, e.g., on the same optical table, which does not easily lend itself to practical application in realistic networks.
While not at wavelengths suitable for long-distance fiber transmission, there have been two recent proof-of-principle experiments that attempt to address the issue by locking laser pulses from two independent lasers (which could in principle be separated). The first approach used a nonlinear Kerr medium for optical pulse locking. This approach is described in “Experimental Synchronization of Independent Entangled Photon Sources,” by Tao Yang, Qiang Zhang, Teng-Yun Chen, Shan Lu, Juan Yin, Jian-Wei Pan, Zhi-Yi Wei, Jing-Rong Tian, and Jie Zhang, Phys. Rev. Lett. 96, 110501 (2006). The second approach used electronic synchronization of two lasers themselves. This approach is described in “Experimental Interference of Independent Photons,” by Rainer Kaltenbaek, Bibiane Blauensteiner, Marek Zukowski, Markus Aspelmeyer, and Anton Zeilinger, Phys. Rev. Lett. 96, 240502 (2006) and in “High-fidelity entanglement swapping with fully independent sources,” by Rainer Kaltenbaek, Robert Prevedel, Markus Aspelmeyer, Anton Zeilinger, Phys. Rev. A 79, 040302(R) (2009). In principle, these approaches would allow lasers in different locations to be used to generate the required entangled photon pairs.
In addition, there has been an attempted solution at telecom wavelengths which uses a special measurement technique and continuous wave (CW) laser pumps, to avoid the need to use the same pulsed pump laser, described in, “Entangling Independent Photons by Time Measurement,” by Matthaus Halder, Alexios Beveratos, Nicolas Gisin, Valerio Scarani, Christoph Simon and Hugo Zbinden, Nature Physics, Vol. 3, 692 (2007).
While interesting, these prior art approaches have a number of shortcomings. For example, the Kerr locking solution requires the pumps from the distant sources to be brought together in one place, locked, and then redistributed to create photons in the two separate locations. The electronically locked solution requires electronic feedback channels to be maintained over long distances. Such a solution fails when the fluctuations that are to be stabilized occur on a time scale comparable to the time it takes for information to travel from distant stations. The CW pump/measurement solution uses exotic low-jitter detectors and very narrow frequency filters to increase the coherence length of the photons, necessarily rejecting most of the resource photon pairs.